Megg

Jaime Griesemer, lead designer of Halo: Combat Evolved, gave his girlfriend Meg (aka Pallor of Subnova) a gift in the form of an easter egg. The egg, affectionately called the Megg, was hidden in Pillar of Autumn (Level) under a series of almost impossible feats to ensure that no one could find it accidentally. The Megg itself is a heart made from human blood, with an "M" made of bullet holes in the center.

After you get the pistol, go through the exit (kill the Grunts) and get the assault rifle, then go back to the bridge.

Kill Keyes or/and whack the crewmen (not sitting down), which will send in the "Invincible Marines" out of a previously locked door, (the right side of the exit).

As Cortana finishes saying, "John-117 has gone rampant," a second door (right behind the first locked door) unlocks.

Move to the other side of the bridge with the locked door and use the assault rifle to keep the Marines at bay. Then skip past through the corridor and head in the second door. (The second door will still be locked, if you go to it right when the Marines come out. Kill Keyes, stay at that position, awaiting the marines to "crowd" one side of the bridge. Then zoom past the "empty" side blazing lead, and into the door way.)

Enter the hallway, at the end you will find the Megg on the ceiling. (Use your flashlight)

There is also a megg egg located in Halo 2. Discovered by de-compiling the game script by "Soldier of Light" from High Impact Halo, Halo 2's version turned out to be much less complicated. It is activated by going through the first level, Cairo Station, on Legendary, without losing your shields once. Upon reaching the final bomb room, a sound clip will play: "Megg, please report to the red courtesy phone. Megg, to the red courtesy phone."